Fudostein is a nonproprietary name for S-(3-hydroxypropyl)-L-cysteine of the following formula (I) and is useful as a drug exhibiting an expectorative effect (Examined Japanese Patent Publication No. 7-88352).

Patients of chronic bronchitis or bronchial asthma have fat respiratory epithelium submucosal glands, secrete an increased amount of epithelium mucus due to excessive formation of goblet cells, and suffer from expectoration difficulties due to excessive secretion of mucin during sickness. For these reasons, these patients have subjective symptoms such as phlegm blocking the peripheral respiratory tract in the breast and difficulty in ejecting phlegm in the central respiratory tract. Fudostein is highly useful as an expectorant, since the drug can significantly improve these symptoms at a clinical dose.
Conventionally, a solid preparation using a starch, for example, as a carrier has been known as a fudostein preparation (Japanese Patent Application Laid-open No.11-35459). However, there has been no liquid fudostein preparation that can be easily taken by a patient of asthma and the like due to a problem of stability.
The reason is because a fudostein solution exhibits change in color and produces sedimentation over time. Although these phenomena have no direct effect on the drug efficacy, the change in color and sedimentation cause anxiety among the patients orally taking the drug and remarkably decrease the commercial value of the drug. Since fudostein has a peculiar taste that gives an unfavorable sensation upon dosing, the fudostein is preferably made into a syrup preparation by adding a sweetener. However, sucrose and glucose commonly used for syrup preparations, if added to a fudostein syrup preparation, react with fudostein and darken the color of the solution. This phenomenon also unduly impairs the commercial value.
Therefore, development of a fudostein-containing liquid drug preparation free from color change and sedimentation during long-term storage can be easily taken has been desired. An object of the present invention is to provide such a liquid drug preparation.